The demand for new and improved electronic devices such as cellular phones, notebook computers and compact camcorders has demanded energy storage devices having increasingly higher specific energy densities. A number of advanced battery technologies have recently been developed to service these devices, such as metal hydride (e.g., Ni-MH), nickel-cadmium (NiCd), lithium batteries with liquid non-aqueous electrolytes and more recently, lithium batteries with polymer electrolytes.
Lithium batteries have been introduced into the market because of their high energy densities. Lithium is atomic number three (3) on the periodic table of elements, having the lightest atomic weight and highest energy density of any room temperature solid element. As a result, lithium is a preferred material for batteries, having a very high energy density. Lithium batteries are also desirable because they have a high unit cell voltage of up to approximately 4.2 V, as compared to approximately 1.5 V for both NiCd and NiMH cells.
Lithium batteries can be either lithium ion batteries or lithium metal batteries. Lithium ion batteries intercalate lithium ions in a host material, such as graphite, to form the anode. On the other hand, lithium metal batteries use metallic lithium or lithium metal alloys for the anode.
The highest specific Li battery characteristics are obtained when a metallic lithium comprising anode, as opposed to a lithium ion anode, is used. However, the use of Li metal comprising anodes for secondary batteries has been limited by certain known technical challenges. A major challenge is the high level of reactivity of lithium metal to a variety of substances, including most common atmospheric materials. For example, lithium metal is known to react with atmospheric gases, such as O2, N2, CO2, H2O and SO2. Reaction of metallic lithium with any of the above reagents can produce compounds which are generally insoluble in the electrolytes used and result in degradation in electrochemical properties of the anode and the electrochemical system as a whole.
Based on a decreasing degree of activity with respect to pure lithium, the common atmospheric substances shown below can be placed in the following order:SO2>O2>CO2>H2O>N2.
The compounds positioned to the left supplant the reagents they are followed by or block their interaction with lithium metal. An exception is water, which generally strengthens the activity of other active atmospheric reagents when present.
It is also known that lithium metal containing anodes can react with certain non-aqueous electrolytes, such as solutions of lithium salts (LiClO4, LiAsF6, LiPF6, etc.) in organic liquids such as propylene carbonate, ethylene carbonate and dimetoxyethane, and produce an alkyl carbonate film on the surface of the lithium anode, which further transforms into carbonate films. The electrolyte salts can also take part in forming these films, for example LiClO4 or LiAsF6, admixtures of water, or carbon dioxide which may be dissolved in the electrolyte. The formation of these films and deposition on the surface of the lithium containing anode when charging the electrochemical cell can result in encapsulation of the anode. Lithium encapsulation can result in the loss of electrical and mechanical contact within the active anode result in a loss of capacity.
Under multiple deep cycling, the electrode structure can become highly disordered which can render the electrochemically active metal substantially inactive. Thus, multiple cycling can result in a significant reduction in capacity of the secondary power source. This situation can be exacerbated by the tendency of lithium to form dendrites due to electrolytic deposition.
The growth of dendrites which penetrate through the secondary battery separator can lead to short circuits between the cathode and anode. System shorts can lead to thermal destruction of the power source. Although electrode films can impede electrode performance, some electrode surface films can provide certain benefits. For example, some surface films can to make the anode and/or the cathode surfaces passive.
When metallic lithium is included in the anode of an electrochemical cell, cathode materials are generally selected which provide low equivalent mass, such as oxides or fluorides of relatively light elements, preferably being in their highest oxidation states. The specific capacity of the cathode material is proportional to the number of electrons participating in cathodic reaction and in reverse proportion to the molecular mass of this material. In most cases, simple and complex oxides of transitional 3d metals are used.
Among the transition metal oxides available, crystalline vanadium oxides are commonly used. Vanadium oxides provide a relatively low equivalent mass and high oxidation states, such as +5 and +4, which allows cathodes to provide high specific capacity and favorable power characteristics to be obtained. Common vanadium oxides include V2O5, V3O7, V4O9, V6O13, VO2(B). For example, V2O5 provides a specific energy density that is generally up to approximately 260 Wh/kg.
Although available vanadium oxide cathodes provide low self-discharge, these materials cannot provide high discharge current characteristics. Moreover, they generally exhibit an undesirable two-stage discharge curve.
Some lithium batteries having vanadium oxide cathodes demonstrate substantially flat discharge curves and may provide high specific characteristics. However, obtainable discharge characteristics have been too low to be useful for most applications because of a rapid decrease in energy density during cycling. A decrease in energy density upon cycling is believed to be primarily caused by the formation of stable lithium vanadates that do not take part in the electrochemical reaction. A similar mechanism of degradation of electrochemical characteristics is also typical for the other known non-vanadium oxide cathode materials.
For example, V6O13 can provide up to 600 Wh/kg of practical energy density for a primary battery. However, this material can only be cycled to a 10% to 15% of depth with respect to a primary battery.
Amorphous oxide V2O5 has been suggested to overcome some of the limitations of the corresponding crystalline vanadium oxide materials. Amorphous V2O5 generally provides a smoother discharge curve in comparison with crystalline vanadium oxide, but does not provide a significant improvement in cycling characteristics.
Thus, the high theoretical characteristics potentially providable by transitional metal oxides cathodes for lithium ion and lithium metal batteries have not been realized by available cathode materials.